1. Field of the Invention
This invention pertains to methods for manufacturing and testing semiconductor photodetector devices and, in particular, to methods for determining photodiode performance parameters including the dynamic impedance-area product R0A, the external quantum efficiency xcex7, the specific detectivity D*, and other photodiode performance parameters.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
It is desirable to employ photodetectors to convert electromagnetic radiation, such as infrared (IR) radiation, into electrical signals. Such photodetectors may be used in a variety of applications, including thermal imaging and transmission of information using signals having infrared wavelengths. One type of photodetector is the junction photodetector, or photodiode, which has a semiconductor p-n junction that produces electrical current under illumination with electromagnetic radiation. When properly biased, therefore, the photodiode thus produces a current related in a known manner to the electromagnetic radiation incident thereon.
The performance of a photodiode may be predicted, to varying degrees of certainty, from various photodiode performance parameters. These performance parameters indicate various properties or characteristics of the photodiode, e.g. its electrical and optical properties. Performance parameters, e.g. normalized performance parameters, may be used as figures of merit, e.g. to compare the operation and characteristics of the device to certain thresholds or to other devices. The terms xe2x80x9cfigure of meritxe2x80x9d and xe2x80x9cperformance parameterxe2x80x9d may be used interchangeably herein.
It is desirable to determine these performance parameters, so as to be able to determine the overall performance of the photodiode or to determine its performance with respect to a particular characteristic. For example, knowledge of the photodiode""s performance may be used for testing a fabricated photodiode during or after manufacture.
The most relevant performance parameters can be assessed according to the ideal diode equation, which for a device under illumination is given by:                               I          =                                                    I                0                            ⁡                              [                                                      exp                    ⁡                                          (                                              qV                        nkT                                            )                                                        -                  1                                ]                                      -                          Iph              ⁢                              xe2x80x83                            ⁢                              (                A                )                                                    ,                            (        1        )            
where I is the photodiode current, I0 is the saturation current, V is the applied bias, n is the ideality factor, k is the Boltzman factor, and T is the operation temperature of the device. As can be seen, the total photodiode current consists of two components. The exponential term represents current contributions arising from diffusion processes in a semiconductor p-n junction, which is sometimes referred to as the dark current. The second term, Iph, is the photocurrent induced under illumination. Because the photocurrent Iph is related to the radiation incident on the photodiode, the total photodiode current I is also related to this radiation. Thus, measuring the current I can provide an indication of the intensity of local radiation.
Referring now to FIG. 1, there is shown a plot of the typical current verses voltage (I-V) curve 100 for an ideal photodiode (not shown). As shown in FIG. 1, under illumination, a zero bias photocurrent, Iph, flows at zero bias. Thus, the short-circuit current of a photodiode is equal to the induced photocurrent Iph. The open circuit voltage, Voc, is the point in forward positive bias where diffusion (dark) current equals the photocurrent so that no net current flows in the device.
The most relevant electrical performance parameter is the dynamic impedance-area product R0A, which is defined as:                                                         R              0                        ⁢            A                    =                                    A              ·                                                (                                                            ⅆ                      I                                                              ⅆ                      V                                                        )                                                  V                  =                  0                                                  -                  1                                                      =                                          nkTA                                  qI                  0                                            ⁢                              xe2x80x83                            ⁢                              (                                  Ω                  ⁢                                      /                                    ⁢                                      cm                    2                                                  )                                                    ,                            (        2        )            
where I is the total diode current from Eq. (1), q is the electron charge, A is the junction area of the device, and R0 is the dynamic impedance at zero bias (i.e., the exponential derivative term in Eq. (2), which is multiplied by area A). This performance parameter embodies the essential elements of the diffusion process in the photodiode junction, and is an industry standard for comparing the electrical performance of photovoltaic structures. R0A is basically an indication of noise: the higher R0A is, the lower the noise. R0A is typically found by measuring the current as a function of voltage (I-V) and calculating the derivative, at V=0, according to Eq. (2).
The most relevant optical performance parameter used to characterize the performance of a photodiode is the external quantum efficiency xcex7. The photocurrent induced in a photodiode of area A due to a background photon flux of QBK can be expressed by:
Iph=xcex7qAQBK,xe2x80x83xe2x80x83(3)
The external quantum efficiency xcex7 is a measure of electrical carriers collected per incident photon, and thus is an indication of signal, ranging from 1(best) to 0(worst). It is typically measured by exciting the device under test (DUT) with a known photon flux within a narrow band around a specified spectral wavelength xcex, measuring the photocurrent, and computing the external quantum efficiency from Eq. (3).
As noted above, the performance of a photodiode is related to these two primary photodiode performance parameters. Specifically, the dynamic impedance-area product R0A is related to its electrical properties (noise), and the external quantum efficiency xcex7 is related to its optical properties (signal), respectively.
Another important performance parameter is the specific detectivity, D*, which is an overall photodiode performance parameter that indicates the signal-to-noise ratio (SNR) for the photodiode. D* is normalized with respect to detector area A and electrical bandwidth. Because the dynamic impedance-area product R0A is an indication of noise, and the external quantum efficiency xcex7 is an indication of signal, D* may be computed from the primary performance parameters, R0A and xcex7. Specific detectivity D* may be referred to herein as an overall performance parameter, because it is based on these two primary performance parameters.
The specific detectivity D* of a photodiode at zero applied bias is given by the expression:                               D          λ          *                =                              q            ⁢                          xe2x80x83                        ⁢            ηλ                                hc            ⁢                                                            2                  ⁢                                      xe2x80x83                                    ⁢                  η                  ⁢                                      xe2x80x83                                    ⁢                                      q                    2                                    ⁢                                      Q                    BK                                                  +                                                      4                    ⁢                    kT                                                                              R                      0                                        ⁢                    A                                                                                                          (        4        )            
where h is Planck""s constant and c is the speed of light. This overall performance parameter is the most widely accepted comparative parameter for specifying the detector""s characteristics and performance. It can therefore be useful to accurately and easily determine the dynamic impedance-area product R0A and the external quantum efficiency xcex7 , so that specific detectivity may be estimated. Additionally, it is sometimes useful to determine the dynamic impedance-area product R0A and the external quantum efficiency xcex7 parameters individually. For example, the external quantum efficiency xcex7 of a given device may be compared to that of other devices or to a benchmark or threshold value. Background information regarding photodiodes and related performance parameters may be found in: Thomas Limperis and Joseph Mudar, xe2x80x9cDetectors,xe2x80x9d Ch. 11 in The Infrared Handbook, rev""d ed., William L. Wolfe and George J. Zissis, eds. (Infrared Information analysis (IRIA) Center, Environmental Research Institute of Michigan, 1985); Semiconductors and Semimetals, vol. 18: Mercury Cadmium Telluride, R. K. Willardson and Albert C. Beer, eds. (New York: Academic Press, 1981), esp. ch. 6, xe2x80x9cPhotovoltaic Infrared Detectors,xe2x80x9d by M. B. Reine, A. K. Sood and T. J. Tredwell; and John David Vincent, Fundamentals of Infrared Detector Operation and Testing (New York: John Wiley and Sons, 1990), esp. ch. 2, xe2x80x9cDetector Types, Mechanisms, and Operation.xe2x80x9d
In addition to R0A, xcex7, and D*, the saturation current I0, dynamic impedance at zero bias R0, and ideality factor n may also be regarded as photodiode performance parameters, because they can be used as figures of merit to compare the performance of the photodiode. For example, the ideality factor n is an electrical performance parameter, and the saturation current I0 is an electrical performance parameter embodying material characteristics. The dynamic impedance at zero bias R0 is also an electrical performance parameter.
There are, however, difficulties in determining these photodiode performance parameters using standard techniques. First, for high-volume production of photodetectors, the amount of experimental data required to extract R0A from I-V measurements is prohibitively large and time-consuming to produce. Second, the electrical and optical properties are typically determined in separate measurements, e.g. the external quantum efficiency must be determined under controlled conditions.
Another, simpler approach, which is not admitted to be prior art by virtue of its inclusion within this section, is to employ an analysis using only two points on the characteristic I-V curve. One of these points is the short-circuit current (i.e., the current measured at zero bias), which is the simply the photocurrent, Iph, produced by the unspecified background photon flux present during the I-V measurement. The other point is the voltage under forward bias for which the diffusion current equals the photocurrent so that no net current flows in the device. When the total photodiode current I is zero, the open circuit voltage can be defined from Eq. (1) as:                     Voc        =                              nkT            q                    ⁢                      ln            ⁡                          (                                                Iph                                      I                    0                                                  +                1                            )                                                          (        5        )            
For unity ideality factors (n=1), the saturation current I0 can also be determined from Eq. (1), as follows:                               I          0                =                  Iph                      [                                          exp                ⁡                                  (                                      qVoc                    nkT                                    )                                            -              1                        ]                                              (        6        )            
and R0A can be estimated directly by definition in Eq. (2).
There are two primary difficulties with this simple two-point analysis. First, the ideality factor, n, is unknown for a given device, and can range from one to three depending on the actual dark current mechanisms present. The assumption that n=1is not always correct. Because the ideality factor is in the exponential, substantial errors can be made in the estimation of R0A if n is not known or imprecisely estimated. Second, the background flux present during the I-V measurement is often not controlled and can vary from measurement to measurement.
The foregoing drawbacks of conventional performance parameter measuring techniques can limit the ability to perform high throughput screening of photodetector performance at low cost. There is, therefore, a need for improved methods for quickly and accurately estimating the primary photodiode performance parameters, which are required for determining the specific detectivity.